import sys
def main():
    def input():
        return sys.stdin.readline()[:-1]
    N = input()
    ans = 0
    L = list(N.split("00"))
    for e in L:
        for k in range(len(e)):
            if e[k] == "1":
                ans += min(e[k:].count("1"),e[k:].count("0")+2)
                break
    print(ans)
if __name__ == '__main__':
    main()